1. Field of the Invention
The present invention relates to composite photography, and more particularly, to projectors used to project light of a desired wavelength in motion picture travelling matte photography.
2. Art Background
In motion picture production, it is sometimes impractical or impossible to place actors in the specific environments to be depicted. To resolve this problem, various techniques have evolved over the years to composite such scenes from separately filmed "elememts." Initially, in early films such as Georges Melies' "Trip to the Moon" (1902), animation was resorted to. Thereafter, techniques were utilized such as the "held/take" process, in which a scene was shot with predetermined areas of the successive frames blocked out in order to preclude exposure thereof. The unexposed portions of the successive frames were thereafter exposed to the desired foreground subjects, with the background areas blocked off with an opaque matte to protect the previously recorded latent images thereon. Essentially the same process is used to incorporate a painting which depicts a distant, dangerous, or totally alien scene against which the actors are to appear; this is known as matte painting.
In order to depict actors or other foreground subjects moving in front of the desired background scenes. various techniques were developed over the years. One early approach involved the so-called "carousel" in which motion effects were simulated, for example, by photographing actors on dummy horses or in automobiles in front of a cylindrical, painted backdrop revolving through the frame behind the actors. This technique evolved into rear projection/front projection filming operations in which a separately filmed background scene was projected onto a screen behind the foreground subject, and re-photographed simultaneously therewith to produce a composite image. (See: Fielding, The Art of Special Effects, pp. 306-323; Alekan-Gerard Process for Composite Photography. U.K. Pat. No. 768.394; Jenkins, U.S. Pat. Nos. 2,727,427 and 2,727,429; Philip Palmquist, American Cinematographer, 7/69; Petro Vlahos, Association of Motion Picture and Television Producers Research Center Report No. R-053 and also Journal of the Society of Motion Picture and Television Engineers, Vol. 80, Sept. 1971; Perisic, U.S. Pat. No. 4,100,572; and Samuelson, American Cinematographer, Oct. 1981.
However, the use of such operations necessitates synchronizing the rear or front projection system and the camera, as well as accurate balancing of both the illumination across the projected image of the background scene and the illumination and color balance of the foreground subject, relative to the background scene. Such techniques are inherently expensive, requiring the prior filming and independent processing of the background scene, and the additional expensive studio time for filming the foreground subject in conjunction with the background plate. There is usually a discernable loss of image quality in the rephotographed projected image (Vlahos/Holm, Association of Motion Picture and Television Producers, Research Center Bulletin No. B-O 30/01) when compared to the original photography of the foreground action.
To retain both image quality and the flexibility inherent in optical compositing, while insuring the freedom of movement basic to the front projection technique, it became necessary to create mattes which would change from frame to frame, or "travel". Such "travelling mattes" were initially produced manually or mechanically. However, in order to meet the ever-increasing demand for fidelity, a wide variety of photographic techniques for producing travelling mattes have subsequently been developed. These generally utilize either the so-called "single-film" or "multi-film" approaches.
Several single-film and multi-film systems of travelling matte cinematography have been described in the patent literature. See, for example, U.S. Pat. Nos. 1,673,019; 1,697,315; 1,840,669; 1,840,670; 1,860,737; 1,863,827; 1,955,993; 2,013,886; 2,028,863; 2,232,144; 2,277,141; 2,297,598; 2,461,127; 2,651,233; 2,693,126; 2,740,712; 3,095,304; 3,149,969; 3,158,477; 3,260,563; 3,902,798 as well as U.S. Pat. No. 4,417,791. The preceeding and like travelling-matte systems have also been reviewed in the technical literature. See, for example, "Travelling-Matte Photography and the Blue Screen System," Beyer, Journal of the Society of Motion Picture and Television Engineers, Mar. 1965, Vol. 74, No. 3, pp. 217-236; "The Technique of Special Effects Cinematography," Fielding, 1968, pp. 218-253; and "Talking Technically," David Samuelson, American Cinematographer, May, 1982, page 434, and June 1982, page 588.
Early single-film matte processes relied upon contrast alone, the foreground action being filmed against a jet black backing and the resulting image being printed through several generations of high contrast film stock until a matte was produced. The results obtained by this technique were generally quite poor, due to the inevitable distortion produced by the multiple reversals.
Subsequently, with the advent of subtractive color film, a single film technique was developed which relied upon the dedication of one of the three emulsion layers of the color negative film to the matte. While all three emulsion layers have been utilized from time to time for matte formation, the blue layer has always been preferred. This is due in part to the relative granularity of the three emulsion layers as well as to the conviction that since flesh tones (which contain very little blue) are normally a part of the foreground subject, the blue record is the more readily sacrificed. In fact, in the hands of a very skillful optical compositor, any of the three records can produce an adequate matte. Nevertheless, the dominant travelling matte process in the industry today is the "blue screen" process.
While there are numerous variations of the blue screen process, each generally involves exposure of a color negative film to a foreground subject in front of a blue backing. The backing may, for example, be a front lit painted backing, a retro-reflective screen which is front lit by reflection from a blue flux projector, or a rear lit translucent blue transmission screen. The color negative image thus formed is utilized, dependent upon the particular technique employed, to produce sets of black and white separation positives and matte images. The mattes are thereafter used to facilitate compositing of the foreground subject with the image of a separate background scene.
Various multi-film systems have also been developed for the formation of travelling mattes. Such typically require the huge, complex "Technicolor "camera, and employ the simultaneous exposure through a beam splitter of a color negative film and a further film sensitive to some narrow portion of the spectrum. The matte image is thereby formed on the latter film, thus eliminating the necessity to produce separation positives, and obviating the loss of any portion of the spectrum for matte formation. Such techniques (See, for example, U.S. Pat. No. 3,095,304) may be utilized in the formation of travelling mattes of objects which move or which have fine details (e.g., hair) without fringing.
However, a major problem with any travelling matte system which utilizes an illuminated screen behind a foreground subject to produce a matte is that the radiation transmitted from the screen, whether it be infrared, ultraviolet or within the visible spectrum, is reflected from any glossy or specular metallic surface on the foreground subject matter causing holes or ragged edges in the mattes. Furthermore, the color negative film currently used for the process, Eastman Kodak No. 5247, requires extremely pure blue light exposure, thus making it very difficult to use blue paint as a backing. This leaves the standard Stewart Transmission blue screen as the backing of choice. But this blue screen backing is limited to a maximum size of 90 feet by 40 feet. An alternative is to use a front projection apparatus and a high gain retro-reflective screen, for which there is no size limit, and project a pure blue light. This technique was probably first done by Abbott on the motion picture Tora!Tora!Tora!. Abbott also noted a serendipitous benefit of great importance obtained by this method of carrying out the blue screen process: blue spill, the bane of blue screen compositing, is largely eliminated by virtue of the fact that the projected blue light is of necessity a coherent beam, and this is further enhanced by the retro-reflective screen's faculty for returning the beam strictly along its own axis. Therefore, there is essentially no scattered blue light remaining to fall upon foreground subject matter from whence it would be reflected back to the camera, with the consequent degradation of the matte image.
The circumstance accompanying Abbott's discovery typifies the problem: his foreground subject consisted of a glistening wet submarine. Had he been required to position the submarine in front of a conventional transmission blue screen, the diffuse and incoherent blue light would have bounced off the wet submarine surface and caused holes in his matte image. A problem that confronted Abbott and subsequent practitioners of this technique is that the white light source-lamps commonly used for front projection of full color plates are relatively deficient in the blue region of the spectrum, requiring the use of very large wattages and commensurately large lamphouses, with the resulting severe inhibition of camera flexibility.
As will be described, the present invention provides a projector having application for use in front projection systems. Although the present invention is particularly suited for use in blue screen travelling matte processes, it has utility in numerous other special effects applications.